This invention relates to integrated circuits, and more particularly, to Complementary Metal Oxide Semiconductor (CMOS) and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) having different gate oxide thicknesses on a semiconductor body (substrate, wafer), and to methods of forming these different thicknesses.
The fabrication of silicon semiconductor wafers has become more complex as designers and process engineers move toward smaller and faster circuit devices. Advanced designs typically use 0.18 xcexcm design rules and semiconductor chips contain more circuit components, such as capacitors and resistors, in addition to transistors. The Integrated Gate Field Effect Transistors (IGFETS) not only require thinner, defect-free gate oxides, but also gate oxides having different thicknesses. Many processes have been developed to obtain these thin gate oxides but, often, the transistors with relatively thin oxides are fabricated in a manner that introduces defects. Many of these defects cannot be removed by annealing and thereby contribute to reliability problems, such as gate leakage or gate oxide breakdown.
The goal that is being sought is to have gate oxides grown by thermal oxidation on major surfaces of the semiconductor body with different thicknesses to provide circuit designers with variable transistor characteristics or performance of one set of transistors with respect to another set of transistors.
One method of fabricating gate oxides that is in use today relies on a photoresist mask to define that part of a major surface of the semiconductor body on which a gate oxide is to be grown. This resist mask method can result in degraded gate oxides when the photoresist is stripped from the xe2x80x98protectedxe2x80x99 gate oxide surface. The unprotected gate oxide is totally removed during the etching so that it is not a concern. It has been shown that when the photoresist is stripped and the gate oxide surface is xe2x80x98cleanedxe2x80x99, residues still remain. These residues get incorporated into the gate oxide during the gate oxide regrowth needed to form multiple gate oxide thicknesses.
Other methods of forming gate oxides found in prior art use ion implantation to effect the silicon surfaces so that subsequent oxide etching of the implanted portion of the gate oxide will be etched more rapidly than on the untreated portion of the gate oxide. However, it is critically important that the implantation energy is controlled so that implantation damage is not a factor during gate oxide regrowth. Some have tried to use low energy implantations to reduce the deleterious effects caused by the high energy implants, but the fine tuning of the implantation energy creates a narrow processing window. This narrow window can limit the effect of the implantation on the etch rate of the implanted portion of the gate oxide and thereby, not completely prevent oxide defects.
It is desirable to be able to create different oxide thicknesses of transistors on a common semiconductor chip while reducing the problems associated with methods used in the prior art.
In a first illustrative embodiment the invention is a method for forming two oxide layers having different thicknesses on a surface of a semiconductor body comprising the steps of: forming a first oxide layer on the surface of the semiconductor body; subjecting a first portion of the first oxide layer to a plasma while protecting a second portion of the first oxide layer from the plasma; subjecting the first oxide layer to an etchant which removes all of the first portion of the first oxide layer which was exposed to the plasma while leaving at least a part of the second portion of the first oxide layer which was protected from the plasma; and oxidizing the semiconductor body to form a second oxide layer on the exposed portion of the semiconductor surface and to increase the thickness of the remaining second portion of the first oxide layer such that the second oxide layer is thinner than the thickened first oxide layer.
In a second illustrative embodiment the invention is a method for forming two oxide layers having different thicknesses on a surface of a semiconductor body comprising the steps of: oxidizing the surface of the semiconductor body to form a first oxide layer thereon; subjecting a first portion of the first oxide layer to a plasma while protecting a second portion of the first oxide layer from the plasma; subjecting the first oxide layer to an etchant which removes all of the first portion of the first oxide layer which was exposed to the plasma while leaving at least a part of the second portion of the first oxide layer which was protected from the plasma; and oxidizing the semiconductor body to form a second oxide layer on the exposed portion of the semiconductor surface and to increase the thickness of the remaining second portion of the first oxide layer such that the second oxide layer is thinner than the thickened first oxide layer.
In a third illustrative embodiment the invention is a method for forming two oxide layers having different thicknesses on a surface of a semiconductor body comprising the steps of: oxidizing the surface of the semiconductor body to form a first oxide layer thereon; providing a protective layer over a portion of the first oxide layer so as to protect same from the effects of plasma; subjecting the partially first oxide layer to a plasma; removing the protective layer; subjecting the first oxide layer to an etchant which removes all portion thereof which was not covered by the protective layer to expose a portion of the surface of the semiconductor body while leaving at least a portion of the first oxide layer which was protected from the plasma by the protective layer; and oxidizing the semiconductor body to form a second oxide layer on the exposed portion of the semiconductor surface and to increase the thickness of the portion of the first oxide layer which was protected by the protective layer such that the second oxide layer is thinner than the thickened first oxide layer.
In a fourth illustrative embodiment the invention is a method for forming two oxide layers having different thicknesses on a surface of a semiconductor body comprising the steps of: thermally oxidizing the surface of the semiconductor body to form a first oxide layer thereon; providing a protective layer over a portion of the first oxide layer so as to protect same from the effects of plasma; subjecting the partially first oxide layer to a plasma; removing the protective layer; subjecting the first oxide layer to an etchant which removes all portion thereof which was not covered by the protective layer to expose a portion of the surface of the semiconductor body while leaving at least a portion of the first oxide layer which was protected from the plasma by the protective layer; and thermally oxidizing the semiconductor body to form a second oxide layer on the exposed portion of the semiconductor surface and to increase the thickness of the portion of the first oxide layer which was protected by the protective layer such that the second oxide layer is thinner than the thickened first oxide layer.
In a fifth illustrative embodiment the invention is two field effect transistors on a common semiconductor body having different thicknesses of gate oxides which are formed using a method comprising the steps of: forming a first oxide layer on a surface of the semiconductor body; subjecting a first portion of the first oxide layer to a plasma while protecting a second portion of the first oxide layer from the plasma; subjecting the first oxide layer to an etchant which removes all of the first portion of the first oxide layer which was exposed to the plasma while leaving at least a part of the second portion of the first oxide layer which was protected from the plasma; and oxidizing the semiconductor body to form a second oxide layer on the exposed portion of the semiconductor surface and to increase the thickness of the remaining second portion of the first oxide layer such that the second oxide layer is thinner than the thickened first oxide layer.
In a sixth illustrative embodiment the invention is first and second semiconductor devices which each comprise a oxide layer formed on a surface of a common semiconductor body with the thicknesses of the oxide layers being different from one another and one of same having been protected from a plasma during fabrication.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawings and claims.